1. Field of the Invention
The present invention relates to switches and, more particularly, to microelectromechanical system (MEMS) switches.
2. Description of the Prior Art
MEMS switches use electrostatic actuation to create movement of a beam or membrane that results in an ohmic contact (i.e., an RF signal is allowed to pass-through) or in a change in capacitance, by which the flow of the RF signal is interrupted.
In a wireless transceiver, p-i-n diodes or GaAs MESFET's are often used as switches, however, these have high power consumption rates, high losses (typically 1 dB insertion loss at 2 GHz), and are non-linear devices. MEMS switches, on the other hand, have demonstrated an insertion loss less than 0.5 dB, are highly linear, and have very low power consumption because they use a DC voltage for electrostatic actuation. If the actuators are coupled to the RF signal in a series switch (i.e., ohmic contact), then the DC bias would need to be decoupled from the RF signal. Usually, the DC current for the p-i-n diodes in conventional switches is handled in the same way. Decoupling is never 100%, and there are always some losses to the RF signal power either by adding resistive losses or by direct leakage.
Another source of losses is capacitive coupling of the actuators to the RF signal, especially when a series switch is closed. If high power is fed through the switch, then a voltage drop as high as 10V can be associated with the RF signal. That voltage is present at the RF electrode of the series switches in the open state. If these electrodes are also part of the closing mechanism (by comprising one of the actuator electrodes), that could cause the switches to close (hot switching) and, thus, limit the switch linearity (generate harmonics, etc.) This is a known problem for transistor switches such as CMOS or FET switches. Thus, to minimize losses and improve on a MEMS switch linearity, it is important to separate entirely the RF signal electrodes from the DC actuators.
Another reason to separate the DC actuators of the switch beam from the RF signal electrode is the need to design single-pole-multi-throw switches for transmit/receive or frequency selection wireless applications. Integrating two or N number of switches in parallel provides a multiple throw switch with N number of throws.
The multi-throw designs are important in commercial wireless applications for multiple frequency and band selection. For example, GSM has typically three frequencies and, thus, a single-pole-four-throw MEMS switch will enable transmit/receive and frequency selection. In addition, if two different protocols are used such as GSM and UMTS, then a double-pole-N-throw switch may be used.
U.S. Pat. No. 6,218,911 B1, incorporated in its entirety herein, describes a lateral MEMS switch and a process of fabrication relying on a single metallization level. A drawback of the lateral switch design described in U.S. Pat. No. 6,218,911 B1 is that the switching element experiences a high level of stress because of the deflection or bending required to close the electrical switch circuit. Such repeated operation of the MEMS switch to more than one billion cycles, will tend to cause fatigue of the metallic materials of the element that are deflected.